1.Field of the Invention
This invention relates to a method of producing a composite metal material, typically a clad steel bloom or slab, comprising outer and inner layers of different compositions, namely of different chemical compositions, and more particularly to such a method wherein the composite metal material is produced by continuous casting.
2. Description of the Prior Art
As methods of producing clad steel materials there are generally known the cast coating method, the explosive bonding method, the rolling pressure bonding method and the overlay welding method.
In the cast coating method, an ingot for the core material is placed in a mold and molten steel of a composition different from that of the ingot is poured into the mold and allowed to solidify, thus producing a clad ingot. Because of its simplicity, this method has been used extensively at steelworks.
However, with the rapid spread of methods for the continuous casting of steel, which are advantageous in terms of production cost, yield and quality, conventional ingoting methods are falling into disuse. This has created a need for methods for producing clad steel materials using continuous casting techniques, and, in fact, a number of such methods have been proposed.
For example, one such method is disclosed in Japanese Patent Publication 44(1969)-27361. In the disclosed method, two immersion nozzles of differing length are inserted into the pool of molten metal in the mold, the outlets of the two nozzles are located at different positions with respect to the direction of casting, and different types of molten metal are poured through the respective nozzles (see FIG. 3 of the present drawings).
In FIG. 3, reference numeral 11 denotes the mold, while 12 and 13 denote the nozzles. The nozzles 12 and 13 are of different length and are used to pour different metals into the mold 11. Reference numeral 14 denotes the pool of molten metal in the mold 11, 15 denotes the outer layer of the composite material and 16 denotes the solidified portion of the inner layer thereof.
In a method that relies solely on using two immersion nozzles for pouring different metals into the mold at different positions, however, regardless of what attempt is made to control the positions at which the different metals are poured into the mold or to control the pattern of the flow of the poured metals, intermixing of the metals will occur between the molten metals in the course of the pouring operation, that is to say, in the course of the continuous casting operation. As a result, the concentration from the outer layer inward of the strand being cast will become uniform in the thickness direction, or the boundary between the outer and inner layers will become extremely indefinite, making it impossible to obtain a composite steel material with the desired sharply defined boundary between the outer and inner layers.
A solution to this problem is proposed in Japanese Patent Publication 49(1974)-44859 wherein, as shown in FIG. 2 of the present drawings, the continuous casting process is carried out using a partition made of refractory material disposed in the mold between the different types of metal
In FIG. 2, reference numeral 21 denotes the mold, and 22 and 23 denote immersion nozzles having different lengths and introducing different metals into the mold 21. Reference numeral 24 denotes a pool of molten metal in the mold 21, 25 denotes the outer layer of a composite steel material, 26 denotes the solidified portion of an inner layer thereof, and 27 denotes a refractory partition.
When a refractory partition of a size large enough to restrict mixing of the different molten metals is introduced into the molten metal pool of the continuous casting strand (the strand pool), however, a major problem arises in connection with the casting operation. More specifically, when the refractory partition comes in contact with the solidifying shell, there is a high risk of its catching on the shell, and as a result a danger either of breaking the refractory partition or of breaking the shell and allowing the molten metal to flow to the exterior of the strand in what is called a "breakout."
Moreover, where the refractory partition in the mold remains immersed in a high-temperature molten metal such as molten steel, problems are apt to arise in connection with its physical strength. Specifically, it is likely to suffer fusion damage or breakage, in which case not only will it become impossible for the refractory partition to fulfill its original purpose but there will also arise serious problems regarding the casting operation and the quality of the product as a result of entrainment of the refractory material in the strand.